Traditionally, underwater communication has often been achieved via two techniques; acoustic communication and optical links (See references [1]-[5]).
Underwater acoustic communication usually involves sending and receiving signals below water using sound waves. There are several ways of effecting such communication; a common way uses hydrophones. A hydrophone may be a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change. Typical frequencies associated with underwater acoustics are between 10 Hz and 1 MHz. A main advantage of using sound waves is that they can travel long distances underwater (up to 20 km) without being significantly distorted. On the other hand, a main disadvantage with this technique is that data cannot be sent with high data rates, and does not fulfill requirements of many applications; additionally, there are problems relating to multipath interference, fading, and long propagation delay (See References [1],[6], and [7]).
Another commonly-used technique for underwater communication involves the use of light waves. Visible light communication (“VLC”) uses visible light between 400 and 800 THz (780-375 nm). A main advantage of VLC is that it can provide higher data rates than acoustic communication, but at the cost of useful transmission distance. Typical ranges for optical modems underwater are in the single meters, and up to tens of meters, if high transmission power is used. This may be partly due to the scattering of light underwater during VLC, as well as the brightness of ambient light, which can be orders of magnitude more intense than the transmitter's signal. Accordingly, to achieve long range VLC, the cost may be quite high. Not only are tens of Watts of additional transmission power needed, but also, the cost of a providing a receiving photodiode with high sensitivity can increase cost up to three orders of magnitude. Furthermore, line of sight between the signal transmitter and receiver is required for communication, as well as high visibility in the water to reduce scattering and increase range.
While acoustics is a preferred modality, since it offers ranges greater than a few meters, optics is still used for wireless sensor networks that can hop short distance to achieve an underwater network.
With respect to the use of electromagnetic (EM) waves in the radio-frequency (“RF”) range for underwater wireless communications systems, underwater communication systems based on EM waves have been proposed before and a few studies tackling this field are available (See References [8]-[11]).
A challenge faced today for underwater wireless RF communications is sending data through RF signals from one point to another without packet loss. RF communication has many advantages, but it suffers significant attenuation when used in underwater communication; for example, wave amplitude decreases rapidly in a short distance.